The present invention pertains to printed wiring boards having printed electrical circuitry on at least one side thereof and components mounted on the opposite side with the leads thereof extending through the board and mechanically and electrically connected thereto by solder or the like at a mounting pad. In printed wiring assemblies of this type cracked solder joints are relatively common because of combined thermal stresses and dynamic environments (shock and other vibrations). Since the board and the connecting lead have significantly different co-efficients of thermal expansion, a stress is set up when the motion of the lead relative to the board is constrained. The amount of stress is proportional to the relative thermal co-efficients of the materials as well as the board thickness and the free lead length of the component. It is well known and has been shown both analytically and experimentally that as the board thickness is reduced the incidence of solder joint cracking is reduced dramatically. For example, in a typical system with a single sided board and constrained component on a .062 inch thick board, solder joint cracking is common and easy to reproduce. With a 0.032 thick board, solder joint cracking is harder to reproduce but visible stress can be induced readily. With a 0.015 inch board, it is difficult, if not impossible to reproduce solder joint cracking and visible stress is relatively minor. Thus, one solution to the relief of thermal cycling solder joint fracture is to use a very thin board (0.015inch or preferably less). However, as the thickness of the printed wiring board is reduced, its ability to withstand dynamic environments, such as sinusoidal or random vibration or shock, is also reduced. Dynamic response of the board goes up and, natural frequency goes down, as the board thickness is reduced. Because of larger excursions of the board at lower frequency resonances, fatigue becomes a problem with solder joints and component leads. Hence, the thin boards must have some means of support other than the board structure. Also, as the area of the board is increased additional supporting structure is required. Hence the use of thinner boards for thermal cycling and the use of thicker boards to meet high dynamic environments appear to be mutually exclusive.
The method most used to combat this problem in the prior art is rigidity of the printed wiring board and flexibility in the component lead. It is not practical to reduce the size of the component leads enough to give the required flexibility so, in most cases, stress relief is gained by increasing the length of the lead and forming some sort of bend such that the forces required to flex the lead are significantly less than those required to break the solder joint. The major drawback of this approach is that it is wasteful of component mounting space. Component mounting density is reduced between 25 and 50% when this method is used over a normal high density board. This waste of space is intolerable in most applications and especially in weight and space sensitive aerospace applications where space or weight is at a premium.